I am trying to prefix a string (reference_)to all the *.bmp files in all the directories as well sub directories.

Correct me if I'm wrong, but you want to prefix the name of the files with "reference_", not the files themselves.

I tried open call with '+<' mode but i am getting compilation error for the read and write mode.

opendir(D,"+<$dir1") wouldn't result in a compilation error. It would result in a run-time error because it won't find the directory you named. "+" and "<" aren't even legal characters for directory names in Windows. readdir. You code has other problems, but I won't go into detail since I'm going to recommend that you use File::Find::Rule or similar.

use strict;
use warnings;

use File::Find::Rule qw( );
use Path::Class      qw( file );

for (
   File::Find::Rule->name( '*.bmp' )
                   ->file()
                   ->in( $dir );
) {
   my $file = file($_);

   my $o = $file;
   my $n = $file->dir()->file(
       'reference_' . $file->basename()
   );

   if (-e $n) {
      warn("$n already exists\n");
      next;
   }

   if (!rename($o, $n)) {
      warn("Unable to rename $o: $!\n");
      next;
   }
}
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